int binsert (bstring b1, int pos, bstring b2, unsigned char fill) * * Inserts the string b2 into b1 at position pos. If the position pos is * past the end of b1, then the character "fill" is appended as necessary to * make up the gap between the end of b1 and pos. Unlike bsetstr, binsert * does not allow b2 to be NULL. */